In this revised A1 application for competitive renewal, we will focus on the impact of HIV-1 replicative fitness as it relates to transmission efficiency and subsequent disease progression. Based on our ten year Uganda/Zimbabwe (Ug/Zim) natural HIV-1 history cohort study involving 270 women (>10,000 data/sample points), CD4 cell declines in subtype C infected women were significantly slower than in subtype A and subtype D infected women. In aim 1, we will continue our analyses of HIV-1 fitness in this cohort and will determine if the pathogenic fitness of the HIV-1 establishing acute/early infection predicts subsequent disease progression based on viral loads and CD4 cell counts. The transmitted/founder (TF) HIV-1 from the blood of ~200 patients will be used in multiple competitions that can now be rapidly analyzed with a new 454 approach and sophisticated algorithms. We now show that slow disease progression is associated with infection by a less fit HIV-1 isolate, generally subtype C whereas subtype D TF HIV-1 isolates have the highest replicative fitness and causes the fastest disease progression. Subtype C may have expanded in HIV pandemic based increased opportunity for transmission (longer disease progression) but relative transmission efficiency between subtypes is the other half of the R0 equation. In aim 2, we will perform infections/competitions using various human mucosal tissue (penile, cervical, and rectal) with a mixture of 5-10 viruses derived from acute/early infection to determine transmission fitness in an ex vivo model. Based on preliminary data, the subtype C TF HIV-1 had higher transmission fitness than subtype A and D TF HIV-1 in the penile and cervical tissue. We also have preliminary data suggesting that HET TF HIV-1 may have different phenotypic properties than subtype B TF HIV-1 derived from men-who-have-sex- with-men (MSM), IV drug users (IVDU), or hemophiliacs (HEMO). In aim 2, we will determine if HET subtype B TF HIV-1 have slightly higher transmission fitness in cervical and penile tissue models than MSM TF HIV-1 or than IVDU/HEMO TF HIV-1. We propose that the penile, cervical, and vaginal mucosa will select for distinct phenotypic properties (aim 3) in the TF HIV-1 from HET and MSM transmission due to a more significant barrier than found in rectal tissue or blood. In specific ai 3, we will the mechanisms controlling pathogenic and transmission fitness. We propose that this pathogenic fitness in PBMCs, T cells, and macrophages is directly related to the efficiency of host cell entry considering that more fit primary HIV-1 isolates have faster entry kinetics, greate ability to infect cells of low CD4/low CCR5 (Affinofile), and are typically less sensitive to entry inhibitors (aim 3). Based on preliminary data, transmission fitness may be more dependent on higher CD4 affinity, reduced levels of N linked glycosylation on the virus envelope, and reduced lectin binding affinity. We propose that the majority of HIV-1 transmitted from donor to recipient is trapped in the mucosal tissue (e.g. cervix) due to lectin binding and other barriers that effectively keep pathogens out.